What % of project managers in a PMO should be Certified?

There is no general set percentage as to how many PM’s should be certified. This depends on the organization itself and what their requirements are.

Jill -

It really depends on what you customers are asking for and whether certification would be a differentiator relative to your competitors.

Organizations who value PM certifications might be more inclined to include this as a requirement within their RFPs for services but I'd say that's the exception in most cases.

Kiron

Saying your organization has a certain percentage of certified PMs can impress customers, but just because a PM is certified doesn't mean s/he is good. You can differentiate your organization from competitors with a wide variety of certifications instead; you could even create an infographic showing how the different certifications help your organization deliver superior service relative to your competitors.
If I were in your position I'd have within my PMO at least one of the following. I know very few organizations that can boast a talent lineup this broad:
Project Management Professional (PMP)
Program Management Professional (PgMP)
PMI Professional in Business Analysis (PMI-PBA)
PMI Agile Certified Practitioner (PMI-ACP)
PMI Risk Management Professional (PMI-RMP)
Certified Six Sigma Black Belt (CSSBB)